AMERICAN MUSEUM NOVITATES Number 3914, 60 pp. December 14, 2018 Bio stratigraphy and Diversity of Paleogene Perissodactyls from the Erlian Basin of Inner Mongolia, China BIN BAI,1-2-3 YUAN-QING WANG,1-2-4 QIAN LI,1-2 HAI-BING WANG,1-2 FANG-YUAN MAO,1-2 YAN-XIN GONG,1-2-4 AND JIN MENG1-5 ABSTRACT Extant perissodactyls (horses, rhinos, and tapirs) comprise a small portion of living mam¬ mals, but fossil perissodactyls were more diverse and commonly dominated Paleogene faunas. Unfortunately, the taxonomy and distribution of some Chinese Paleogene perissodactyls remain controversial and unclear, hampering the correlation of Asian paleofaunas with paleofaunas from other continents. Here we clarify the temporal and spatial distribution of Paleogene perissodactyl species from the Erlian Basin based on published specimens, archives, and our recent fieldwork. The strata of the Erlian Basin range nearly continuously from the late Paleocene to the early Oli- gocene, and almost all Eocene Asian Land Mammal Ages (ALMA) are based on corresponding faunas from the Erlian Basin. We revise the most complete section of deposits at Erden Obo (= Urtyn Obo) that range in age from the late Paleocene to the early Oligocene in the Erlian Basin, and correlate it with other type formations/faunas in the basin based mainly on the perissodactyl bio stratigraphy and lithostratigraphy. Furthermore, we discuss perissodactyl faunal components and their diversity from the early Eocene to the early Oligocene in the Erlian Basin, as well as the 1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing. 2 CAS Center for Excellence in Life and Paleoenvironment, Beijing. 3 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing. 4 College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing. 5 Division of Paleontology, American Museum of Natural History, New York. Copyright © American Museum of Natural History 2018 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3914 correlation between middle Eocene ALMAs and North American Land Mammal Ages based on perissodactyl fossils. The general decrease in perissodactyl diversity from the middle Eocene to the late Eocene can probably be attributed to a global climatic cooling trend and related environ¬ mental changes. The diversity of perissodactyls declined distinctly during the Eocene-Oligocene Transition, when global average temperatures dropped considerably. INTRODUCTION The Erlian Basin is located in central Inner Mongolia (Nei Mongol), near the China-Mongo- lia border (Jiang, 1983; Wang et al., 2012). Deposits of the Erlian Basin range nearly continuously from the late Paleocene to the Oligocene, and produce a large number of fossil mammals and other vertebrates. The lithostratigraphic units in the Erlian Basin include the Houldjin, Arshanto, Irdin Manha, Tukhum, Ulan Shireh, Shara Murun, Ulan Gochu, and Baron Sog formations, named by the Central Asiatic Expedition (CAE), as well as the Nomogen Formation and other lithological units that were proposed by later researchers (Jiang, 1983; Wang et al., 2012). The basin was first investigated by the Russian geologist V.A. Obruchev in 1892, and then during the 1920s, the CAE of the American Museum of Natural History extensively explored the area over five field seasons (fig. 1) (Andrews, 1932; Berkey and Morris, 1927). After the CAE suspended its work after 1930, the Sino-Soviet Paleontological Expedition (SSPE) conducted investigations in 1959-1960 the fossiliferous deposits of the Erlian Basin (Chow and Rozhdestvensky, 1960), followed by the Regional Geological Survey of the Nei Mongol Autonomous Region (1960s-1970s) (Jiang, 1983) and the Institute of Vertebrate Paleontology and Paleoanthropology, in collaboration with the American Museum of Natural History and Carnegie Museum of Natural History, from the 1980s to the present time (Meng et al., 2007; Wang et al., 2010; Jin, 2012; Wang et al., 2012). Derived from that extensive work, the Eocene Asian Land Mammal Ages (ALMA) were estab¬ lished based mainly on the mammalian faunas from the Erlian Basin (Romer, 1966; Tong et al., 1995; Wang et al., 2007; Vandenberghe et al., 2012). The Eocene mammalian faunas of the Erlian Basin are dominated by diverse perissodactyls (Tong et al., 1995; Wang et al., 2007). Perissodactyls from the Erlian Basin consist of members of Tapiroidea, Rhinocerotoidea, Brontotheriidae, and Chalicotherioidea. Since the first report on perissodactyl paraceratheres from Houldjin (Osborn, 1923a), numerous studies, including many papers and some monographs, have contributed to our knowledge of perissodactyls based on the abundant, relatively complete material from the Erlian Basin. The brontotheres were reviewed by Granger and Gregory (1943) and Mihlbachler (2008) on the basis of numer¬ ous, complete skulls in the CAE collection. Wang (1982) described a skeleton of the brontoth- ere Rhinotitan mongoliensis in detail based on material in the SSPE collection from Ula Usu. Radinsky (1965, 1967) studied the tapiroids and hyracodontids from the Paleogene of Asia based principally on the CAE collection. Xu (1966) analyzed many amynodont fossils collected by the SSPE from Inner Mongolia. Qiu and Wang (2007) reviewed the Asian Paraceratheriidae and studied a nearly complete skeleton of Juxia from Ula Usu. However, some perissodactyls from various localities in the Erlian Basin, particularly specimens in the CAE collection, were reported from horizons and formations that had been misinterpreted because of the complexity 2018 BAI ET AL.: BIOSTRATIGRAPHY AND DIVERSITY OF PALEOGENE PERISSODACTYLS 3 FIG. 1. Paleogene fossil localities in the Erlian Basin of Inner Mongolia, China, and the related routes of Central Asiatic Expeditions during 1920s (modified from Jiang, 1983; Mao and Wang, 2012; Wang et al., 2012). 1, Houldjin; 2, Arshanto; 3, Irdin Manha; 4, Daoteyin Obo (= Overnight Camp, 5 mi east of Camp Margetts); 5, Duheminboerhe (= Camp Margettes); 6, Nuhetingboerhe (= 6 mi west of Camp Margetts); 7, Wulanboerhe; 8, Huheboerhe (- 7 mi west and southwest [235°] of Camp Margetts); 9, Chaganboerhe (= 10 mi southwest of Camp Margetts); 10, Bayan Ulan; 11, Nom Khong (= Holy Mesa); 12, Wulantaolegai (= Viper Camp, 4 mi north of Tukhum Lamasery); 13, Wulanhuxiu (= Chimney Buttes, 8 mi north of Tukhum Lamasery); 14, Erden Obo (= Urtyn Obo); 15, Ganggan Obo (= Ulan Shireh Obo); 16, Heretu (= Spring Camp); 17, Bayan Obo (= Twin Obos); 18, Jhama Obo; 19, Xilin Nor North (= 4 mi north of Baron Sog Lamasery); 20, Ulan Gochu (= 8 mi north of Baron Sog Lamasery); 21, Ula Usu. The black-and-white dashed line represents railway. of correlating terrestrial stratigraphic sections. For instance, the names “Irdin Manha Forma¬ tion” and “Houldjin Formation” applied by the CAE in the Huheboerhe area (Camp Margetts and neighboring areas) are mostly considered today to be Arshanto and Irdin Manha forma¬ tions, respectively (Meng et al., 2007). On the other hand, most CAE specimens fortunately retain an association between their field numbers and related field notes, which indicate more specific, accurate layers in the local stratigraphic sections (Meng et al., 2007). Over the past decade, we have recovered many new perissodactyl fossil specimens in the Erlian Basin, and detailed studies of most of them are ongoing. Here, we clarify the distributions of the Paleogene perissodactyls at specific levels from the Erlian Basin based on published specimens, archives, 4 AMERICAN MUSEUM NOVITATES NO. 3914 and the preliminary results of our recent fieldwork (appendices 1, 2). In this paper, we focus on biostratigraphic and lithostratigraphic correlations rather than the morphology of peris- sodactyls from the Erlian Basin. Additionally, a revised correlation of the Eocene ALMAs with their corresponding NALMAs is proposed based mainly on perissodactyl fossils. It also worth noting that a decrease in perissodactyl diversity from the middle Eocene to the early Oligocene likely can be attributed to a cooling trend of global climatic change over that period of time. MATERIAL AND METHODS The perissodactyl fossils from the Erlian Basin are housed mainly at the Division of Pale¬ ontology, American Museum of Natural History (AMNH), and in the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP). A few CAE collections also are curated in the Field Museum (but sent by AMNH), and some CAE fossils that were sent to the Chinese Geo¬ logical Survey Museum are missing. The Paleontological Institute of the Russian Academy of Sciences also houses some SSPE specimens from the Erlian Basin. We checked field numbers of CAE specimens as recorded in Granger’s CAE field notes that are stored in the AMNH Library. Those field numbers usually indicate specific and accurate horizons where the specimens were collected, contrasting with the lithological formations that sometimes have been misinterpreted or are otherwise misleading. Although some beds or formations recorded in the field notes were misinterpreted, we still use their original informa¬ tion, denoted with quotation marks. For example, the fossil-bearing layers from Wulanhuxiu originally were interpreted as “Shara Murun beds” in 1925, rather than the Ulan Shireh Forma¬ tion. In some cases, some specimens’ field numbers were not recorded in Granger’s field notes,